This invention relates to digital protective relay systems, and more particularly to digital protective relay systems of the type including a plurality of digital operation units.
Applications of microcomputers to protective relay systems for protecting electic power systems are well known. An example of such prior art digital protective relay systems is shown in FIG. 1. In this example, a plurality of data on the power system such as those of voltages v and currents i derived from the power system are applied to input converters 1-1, 1-2, . . . , 1-6 which convert the data into voltage and current values suitable for the subsequent processing. The outputs of the input converters are then applied to sample/hold (S/H) circuits 2-1, 2-2, . . . , 2-6, by which simultaneous sampling of all the inputs are made. The outputs of the sample/hold circuits 2-1 through 2-6 are applied to a multiplexer (MPX) 3, whose outputs are serially applied to an analog/digital (A/D) converter 4 and converted into digital values. The thus digitized data or relay inputs are then applied to a digital processing device 8. In the digital processing device 8, the digital values from the A/D converter 4 are supplied through a direct memory access (DMA) controller 5 to a digital operation unit or central processing unit (CPU) 6. The CPU 6 comprises an arithmetic logic unit (ALU) 6-1, a data memory (MEM) 6-2, a program memory (ROM) 6-3, and an output (DO) circuit 6-4. The ALU 6-1 receives the above described voltage data and current data, in digital form, supplied from the DMA controller 5 through the data memory (MEM) 6-2 as well as set values from a setting circuit (SET) 7. The ALU 6-1 carries out discriminating operations of the respective relaying elements on the basis of the data supplied thereto and according to predetermined programs stored in the ROM 6-3. The ALU 6-1 then performs sequential processing, i.e., logical operation to determine the synthetic result by logically combining the results of the discriminating operations and delivers a final output i.e., a relay output RY through the output circuit 6-4 in accordance with predetermined programs stored in the ROM 6-3.
FIG. 2 illustrates well-known operational characteristics of a conventional distance relay, in which three relay elements, i.e., a mho element SU and reactance elements 01 and 03 are combined.
FIG. 3 is a block diagram showing a logic circuit capable of providing the operational characteristics shown in FIG. 2. The logic circuit comprises logical-product circuits (AND gates) AND1 and AND2, a logical-sum circuit (OR gate) OR, and timers TDE1 and TDE2, and delivers the relay output RY.
According to the recent development in semiconductor techniques, it has been made possible to use, as the processing unit 6, a microprocessor which is operable at a high speed. By use of the microprocessor operable at a high speed, a protective relay system capable of performing discriminating operations of a large number of relay elements and capable of performing complicated logical operations can be realized.
Furthermore, the recent increase in the size of the power system has led to expansion of protective relay installations required for protecting the power system, which is problematical. As a solution to such a problem, digital protective relay systems are increasingly employed, which is capable of fault discriminations for a large number of objects to be protected, and of complicated logical operations on the results of the discriminations and which can be made compact.
The prior art digital protective relay system which includes a single digital operation unit (CPU) adapted to perform fault discriminations of a large number of objects to be protected, and a number of logical operations, has the following drawbacks.
For executing the fault discriminations of various objects by use of a single digital protective relay system, not only is it necessary for the microprocessor and the peripheral circuits thereof to be operable at a high speed, but also the circuit elements for detecting voltage data v and current data i need to operate at a high speed. As a result, the cost of the entire protective relay system is increased. Furthermore, the use of such a high-speed relay system would be excessive when the number of objects to be protected is small, and hence a small number of fault discriminations are required. It is thus difficult to provide a protective relay system whose capability matches the particular need.
In addition, the above described relay system exhibits a difficulty in expanding its capability. For instance, when an electric power transmission line is expanded and the number of objects to be protected increases, it is not sufficient to increase the number of the input circuits for inputting the system data such as data on voltages v and currents i, and the number of control circuits for the input circuits, but the programs for executing the protection of the transmission line must be also modified. After the modification of the programs, not only those parts of the programs which have been added, but also the entire programs need to be checked as to their function for assuring the reliability of the programs.
For this reason, the expansion in capability of the protective relay system to cope with an increase in the number of the objects for the protection is not easy. Furthermore the provision of expansibility of the system also encounters a difficulty from the designing and the construction of the system. More specifically, a system so designed as to allow for a large number of input data (voltage data v and current data i) would have an excessive capability when the relay system is applied to a power system where the number of the input data is small. Furthermore, since different programs must be formed for different numbers and combinations of the objects to be protected, standardization of the programs is difficult. Moreover, since the entire operations are executed by a single digital operation unit, a trouble which has occurred in a part of the function of the digital operation unit may cause break-down of the entire operations. The operation of the relay system is therefore not reliable.
Thus, although the conventional circuit arrangement may be applied to form a digital protective relay system for a large number of objects to be protected, such a system has various problems.